The present invention is directed, in general, to wireless communications systems and, more specifically, to cancellation of interference from wireless communications within multipath channels.
In wireless channels, where signals can arrive at different times following different paths, the received signals can experience large amplitude and phase variations due to the interference of the different paths. This phenomena is referred to as multipath fading, and the effect is a critical parameter for consideration in receiver design. For terrestrial mobile telephony, multipath fading may dictate the entire system capacity and throughput rate.
Terrestrial wireless signals transmitted from or to a mobile station may be reflected from the terrain, fixed or mobile objects in the propagation path such as buildings or vehicles, or from a discontinuity in the atmosphere. If the energy of the reflected wireless signal is not significantly absorbed and/or attenuated, creating a plurality of different propagation paths for the wireless signals between the transmitter and receiver, referred to as multipath propagation, which allows the wireless signals to xe2x80x9cbendxe2x80x9d around corners and propagate beyond terrain features and objects obstructing the line-of-sight between the base and mobile stations.
Three problems associated with multipath propagation for mobile stations include (1) the delay spread of the received signal, (2) the Rayleigh fading in received signal strength caused by varying phase shifts between different paths, and (3) the varying frequency modulation due to the Doppler shift between various propagation paths. The fact that propagation paths for reflected signals are longer than the direct propagation path from the transmitter to the receiver (e.g., from the base station to the mobile station) gives rise to signal delays and, because various paths lead to slightly different arrival times, the received signal xe2x80x9cspreads.xe2x80x9d Rayleigh fading results from differences between the phase and amplitude of the reflected wireless signals relative to the phase of a directly propagating signal, attenuating the signal strength at the receiving end (e.g., reception of two signals propagated along two different paths and arriving with a phase difference of 180 degrees results in cancellation in the receiver). Doppler shift is caused by the movement of the mobile stationxe2x80x94or a vehicle or other reflecting objectxe2x80x94in relation to the base station, such that the mean frequencies of both the received reflected signal and of the directly propagated signal deviate from the mean frequency of the transmitted signal by a different amount and in a different direction.
In general, multipath fading causes wide variations in received signal amplitudes, and much effort has been expended in attempting to mitigate the impact of multipath fading. More specifically, multipath fading limits the signal strength of the pilot channel as a function of total interference density in the carrier band for contemporary systems, requiring the largest percentage of transmitted power to be allocated to the pilot signal.
Moreover, while interference cancellation for an unmodulated pilot channel is straightforward, cancellation of xe2x80x9cself inter-ferencexe2x80x9d in the data bearing channel when transmit power allocated to the data bearing channel exceeds that allocated to the pilot channel is substantially more complicated.
There is therefore a need in the art for a technique of eliminating multipath interference from wireless signals received via a multipath channel without regard to whether the interfering signals are unmodulated or modulated.
To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide, for use in a wireless communications system, a wireless receiver which stores one or more frames of received data and, during an initial demodulation, resolves each individual path for the multipath channel and estimates (pilot) channel parameters in order to process the received traffic, then preferably decodes the processed traffic to produce an estimate of the transmitted data. The estimated data is re-encoded exactly as encoded by the transmitter, then scaled and time-shifted utilizing complex weights and timing estimates garnered during the initial demodulation process. For each resolvable path, all other paths are time-aligned to reproduce the interference detected in the original received signal, with the resulting information employed to cancel the interference from other paths. Once all paths have had interference from other paths canceled, the results are combined and again decoded, re-encoded and reprocessed iteratively until residual interference and/or decoding errors are eliminated.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.
Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms xe2x80x9cincludexe2x80x9d and xe2x80x9ccomprise,xe2x80x9d as well as derivatives thereof, mean inclusion without limitation; the term xe2x80x9cor,xe2x80x9d is inclusive, meaning and/or; the phrases xe2x80x9cassociated withxe2x80x9d and xe2x80x9cassociated therewith,xe2x80x9d as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term xe2x80x9ccontrollerxe2x80x9d means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases.